Thursday, 5 March 2015

Mapping Mountains – Trimble Surveys – Moel Hebog


28.02.15  Pt. 36.9m, Ynys Hir (SH 566 398, bwlch only)

Thirteen months ago Aled took me to a magical land of small wooded islands, all situated around Porthmadog, with most having been tidal islands with their land area dictated by the inward wash of the sea.  The first of these islands we had visited was Ynys Hir, in affect this name is given to two hills, both of which are next to the A487.  I’d passed these hills on many occasions travelling between mid and north Wales, but I had never thought to investigate their inner realms.  On that first island adventure the Trimble had surveyed the respective summits but not the connecting bwlch.  The data produced on that day complimented that from the Ordnance Survey Historical 1:25,000 map and not that from current maps, which do not include an uppermost 30m ring contour as the two Ynys Hir summits were surveyed as 37.3m (SH 566 396) and 36.9m (SH 566 398) respectively.

Having visited Ynys Galch (SH 567 393) and Ynys Pen Syflog (SH 561 395) earlier in the day we parked in a large lay-bi beside the wooden gate that gives access into the island realm.  As soon as one enters this land the atmosphere changes from that of busy road and overhead sky to a labyrinthine land of trees and undergrowth, all overshadowed by rising wooded slopes and impressive small crags.

A path leads from the gate upto the bwlch between the two Ynys Hir hills, bisecting the bwlch is a solid stone wall, on the other side of which is a manicured green lawn adjoined to buildings named Bodawen.  The contrast between one side of this wall and the other is immediate as within the island territory the land consists of overgrown brambles, fallen trees and a snaking growth of ivy, whilst above are the islands and a thick canopy of mature trees.

The critical bwlch was judged to be a couple of metres from the wall and the remnants of a rolled up wire fence proved ideal to lay the Trimble on so it was at least partly elevated above the undergrowth.  Soon we had also found the remains of a broken glass cabinet which when positioned on top of the rolled up wire fence elevated the Trimble 1.09m above the bwlch.

Gathering data at the critical bwlch

Measuring the offset between the ground at the bwlch and the internal antenna
As the equipment rhythmically beeped each of its gathered data points we stood under a tree and chatted, 14 minutes later the Trimble was switched off and we delicately made our way over a fallen tree and back onto the path.

A few minutes later and I was driving toward Nantlle with the first spots of rain falling, and once at my brother’s house the wind howled across the land and the rest of the afternoon the rain fell.


Survey Result:


Pt. 36.9m, Ynys Hir

Bwlch Height:  11.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 56713 39759

Drop:  25.8m (converted to OSGM15) (30-99m Sub-Twmpau status confirmed)

Dominance:  69.90%



To read the blog post on the survey of this hill’s summit please click {here}

For further details please consult the Trimble survey spreadsheet click {here}


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